Using a relativistic transport model for the expansion stage of S+Au collisions at 200 GeV/nucleon, we show that the recently observed enhancement of low-mass dileptons by the CERES Collaboration can be explained by the decrease of vector meson masses in hot and dense hadronic matter. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.

Enhancement of low-mass dileptons in heavy ion collisions.

The role of nucleon structure in finite nuclei

Nucleon and hadron structure changes in the nuclear medium and the impact on observables

The change of hadron properties in dense matter based on various theoretical approaches are reviewed. Incorporating these medium effects in the relativistic transport model, which treats consistently the change of hadron masses and energies in dense matter via the scalar and vector fields, heavy-ion collisions at energies available from SIS/GSI, AGS/BNL and SPS/CERN are studied. This model is seen to provide satisfactory explanations for the observed enhancement of kaon, antikaon and antiproton yields as well as soft pions in the transverse direction from the SIS experiments. In the AGS heavy-ion experiments, it can account for the enhanced ratio, the difference in the slope parameters of the and transverse kinetic energy spectra and the lower apparent temperature of antiprotons than that of protons. This model also provides possible explanations for the observed enhancement of low-mass dileptons, phi mesons and antilambdas in heavy-ion collisions at SPS energies. Furthermore, the change of hadron properties in hot dense matter leads to new signatures of the quark - gluon plasma to hadronic matter transition in future ultrarelativistic heavy-ion collisions at RHIC/BNL.

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Medium effects in high energy heavy-ion collisions

The quark-meson coupling model, based on a mean-field description of nonoverlapping nucleon bags bound by the self-consistent exchange of $\ensuremath{\sigma}$, $\ensuremath{\omega}$, and $\ensuremath{\rho}$ mesons, is extended to investigate the change of hadron properties in finite nuclei. Relativistic Hartree equations for spherical nuclei have been derived from a relativistic quark model of the structure of bound nucleons and mesons. Using this unified, self-consistent description of both infinite nuclear matter and finite nuclei, we investigate the properties of some closed-shell nuclei and study the changes in the hadron masses of the nonstrange vector mesons, the hyperons, and the nucleon in those nuclei. We find a new, simple scaling relation for the changes of the hadron masses, which can be described in terms of the number of nonstrange quarks in the hadron and the value of the scalar mean field in a nucleus.

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Variation of hadron masses in finite nuclei

An explicit quark model, based on a mean field description of non-overlapping nucleon bags bound by the self-consistent exchange of $\sigma$, $\omega$ and $\rho$ mesons, is used to investigate the properties of both nuclear and neutron matter. We establish a clear understanding of the relationship between this model, which incorporates the internal structure of the nucleon, and QHD. Finally, we use the model to study the density dependence of the quark condensate in-medium.

K. Saito

Papers

A quark-meson model for nuclear and neutron matter